Some modern communication standards have the capability of transmitting information with a variable data rate. One example of this is the Bluetooth communication standard. In this standard, various types of modulation are provided for transmitting various data rates. For a data transmission rate of 1 Mbit/s, frequency shift keying (GFSK modulation) is used as a type of modulation. For medium and high data transmission rates of 2 to 3 Mbit/s, a π/4 DQPSK and 8 DPSK modulation, respectively, are provided as types of modulation. Whereas in pure frequency shift keying, information is only transmitted over the time of a zero transition, an amplitude and a phase of the signal are changed simultaneously in the two π/4 DQPSK and 8 DPSK types of modulation which produces different requirements for a receiver.
FIG. 5 shows a typical block diagram of a receiver system for such a mobile communication standard. The received signal with a frequency fRF is amplified in a radio-frequency input stage 1 with a low-noise amplifier 12 and converted to an intermediate frequency fIF by means of a mixer 13. For this purpose, the mixer 13 uses a local oscillator signal with the frequency fLO. The signal converted to the intermediate frequency fIF is supplied to a complex channel filter 2 which is arranged as a band-pass filter.
The filtered signal is amplified in a signal conditioning circuit 3 and digitized in a downstream analog/digital converter. In the signal conditioning circuit 3, the filtered signal is amplified up to a level which is suitable for the subsequent analog and digital signal processing. For example, the resolution of the downstream analog/digital converter is utilized by the gain set. The receiver path presented here contains a number of distributed amplifier stages having, in each case, individual gain factors which result in a common gain factor.
Depending on the mobile radio standard used, the gain factors in the individual stages are designed differently for optimum reception. For example, in the case of pure frequency modulation in which frequency shift keying is used, it is sufficient to work with limiting amplifier stages since there is no information contained in the signal amplitude. The amplifier stages can be operated, therefore, in limiting mode. Higher-valued modulation methods such as the π/4 DQPSK and 8 DPSK method described, however, also use amplitude and phase information. The amplification of a signal modulated with such a modulation method therefore requires a linear transfer characteristic of the amplifier stages.
To improve the signal/noise ratio of the received signal further, it is suitable to amplify the signal greatly, as far as possible before any complex signal processing. However, it is important to note that high input levels of a received signal are also amplified linearly so that any amplitude information which may be present is not corrupted. For this reason, modern communication systems use active control of the amplification in which, for example, the level of the input signal is determined and its amplification is adjusted in dependence thereon. The associated power measurement, called RSSI (radio signal strength indicator) measurement, allows active control.
A particular problem occurs with a mobile radio standard which changes the data rate/type of modulation variably during a transmission. Such an example is the new version of the Bluetooth mobile radio standard which operates in packet mode. In this mode, header and packet information, in particular, is first transmitted in data packets with a low GFSK data rate and GFS modulation and then the payload data are transmitted with the same or a medium or higher data rate with π/4 DQPSK or 8 DPSK modulation. It is thus necessary to determine the receive level of the received data packet and from this to suitably adjust the gain factor in dependence on the type of modulation of the payload data in order to prevent amplitude or phase errors.
Accordingly, a need exists for a simplified method for determining a suitable gain factor, as well as a corresponding signal conditioning circuit and method of use thereof.